1. Technical Field of the Invention
The present invention relates generally to a starter for automotive engines and an electric rotary machine designed to have an improved structure to withstand vibrational impacts.
2. Background Art
Japanese Patent Publication No. 3125944 discloses a starter equipped with an output shaft to which torque of a motor is transmitted through a speed reducer, a clutch joined to an outer periphery of the output shaft through helical splines, and a pinion gear fitted on the output shaft integrally with the clutch. When it is required to start an engine, the pinion gear is shifted away from the motor together with the clutch into mesh with a ring gear of the engine to output the torque, as transmitted from the clutch, to the ring gear to crank the engine.
The motor has an armature shaft which is retained by a sleeve bearing and not held from moving in an axial direction thereof. Therefore, when subjected to pulsations of torque produced by the engine being cranked, the armature rotates while vibrating in the axial direction. In order to avoid such a problem, as illustrated in FIG. 19, the armature shaft 410 of the starter has the small-diameter end 420 fitted in the sleeve bearing 400 in abutment of the shoulder 430 with the end of the sleeve bearing 400 to hold the armature shaft 410 from moving in the rightward direction, as viewed in the drawing. A thrust load acts between the shoulder 430 of the armature shaft 410 and the end of the bearing 400. An increase in pressure exerted on surfaces of the shoulder 430 and the end of the bearing 400 will result in wear or seizing thereof. In order to alleviate this problem, the flange 440 which extends radially and outwardly is formed on the end of the bearing 400 to increase an area of the bearing 400 which is in contact with the shoulder 430 of the armature shaft 410 for decreasing the pressure per unit area acting on the shoulder 430 and the bearing 400.
The increase in area of contact between the shoulder 430 and the end of the bearing 400 may also be achieved by 1) increasing diameter of the end 420 of the armature shaft 410 to increase the size of the bearing 400 or 2) decreasing the diameter of the end 420 while increasing the thickness of the bearing 400. The wear or seizing of the surfaces of the shoulder 430 and the end of the bearing 400 may be avoided by installing a washer therebetween to decrease speeds of the shoulder 430 and the bearing 400 relative to the washer.
The bearing 400 with the flange 440 is usually required to be machined in a special manner, thus resulting in an increase in manufacturing cost of the starter. Additionally, the thrust load usually concentrates on the flange 440, thus increasing ease of breakage of the flange 440.
The increase in diameter of the end 420 of the armature shaft 410 to increase the size of the bearing 400 requires an undesirable increase in diameter of the armature shaft 410 to secure a desired area of the shoulder 430, thus resulting in an increased entire weight of the armature. This is a very serious problem for modern starters required to be lightweight. The increased size of the bearing 400 requires a change in design of a frame retaining the bearing 400, which leads to an increase in total production cost of the starter.
The decrease in diameter of the end 420 of the armature shaft 410 to increase the thickness of the bearing 400 may result in a lack of mechanical strength of the end 420. This method is, therefore, unuseful. Additionally, an increase in thrust load acting on the bearing 400 will result in a loss of torque of the armature shaft 410, thus decreasing the output of the starter. It is, thus, essential to decrease the thrust load acting on the armature shaft 410.